Military communication planning is placing increased emphasis on broadband operation. This is particularly true in the high frequency (HF) communication spectrum. Planners are looking towards spread-spectrum and frequency-hopping systems to meet their anti-jamming and low probability of intercept requirements. Because HF is a four octave (2 to 30 Mhz) spectrum, radio frequency changes require large broadband antennas or, where antenna sizes are limited, automatic antenna couplers with near-instantaneous re-tuning capabilities. Additionally, these antennas must not adversely effect the aerodynamics of the aircraft.
Resonant antennas are used when size is a limitation. Wave antennas would be preferable for spread-spectrum and frequency hopping because they are inherently broadband, but conventional versions are several wavelengths long. At the highest HF frequency a five wavelength wave antenna would be 50 meters long and it would be 750 meters long at the lowest HF frequency; far too long for most airborne, ground vehicular, and many fixed shore installations. The specific difference between a wave antenna and resonant antenna is that the incident wave of a resonant antenna is reflected back from its end. The reflected wave sums with the incident wave to form a standing wave and a sinusoidal current distribution results. On a wave antenna, the incident wave is terminated in a characteristic impedance, either by radiation or in a matched resistive load. No reflection results in a traveling wave, constant current distribution along the conductor, and broadband frequency response, long, wire antennas terminate the incident wave by radiative loading when they are greater than five wavelengths. Helical antennas will exhibit constant current characteristics at five wavelengths when the circumference of the helix approaches one wavelength. Conventional wave antennas such as terminated or inverted Vee shape, rhombus and beverage versions, require several wavelengths per leg and usually terminate in non-reactive resistor networks.
The two main characteristics of wave antennas which are important to the needs described above are due to the constant current distribution. First of all constant current distribution provides the highest current-per-unit-length of radiator for a given antenna size. This provides the maximum radiated field intensity for a given transmitter power and antenna size. Secondly, a constant current distribution provides broadband frequency response. When the transmitter output impedance is matched to a constant current antenna the Voltage Standing Wave Ratio (VSWR) will remain low throughout the bandwidth of the termination and matching network. Also, termination and matching networks are amenable to broadband design without being deliberately resistive. The radiation patterns of wave antennas are affected by input frequency changes; however, this is not critical in compact wave antenna applications because the radiation pattern would be dominated by the relatively larger size of the vehicle or airframe portion of the radiating system.
Thus, to achieve practical success depends upon the required operating bandwidth, the RF characteristics of the platform structure within that bandwidth, the VSWR limits of the transmitter, and how well the compact wave antenna design maximizes the efficiency of the radiative termination within its allotted size. When bandwidth requirements are not critical and modern auto-tune couplers are employed requirements can be met and performance improved with smaller, semi-terminated compact wave antennas. Here, the compact wave antenna would be terminated sufficiently to reduce the reflective wave, improve radiation efficiency and improve the probability of successful coupler tuning.
Thus, it is a primary object of the subject invention to provide a compact wave antenna which is lightweight, conformal with low parasitic drag.
It is another object of the subject invention to provide a compact wave antenna which maximizes the aperture of the radiating system.
It is a further object of the subject invention to provide a compact wave antenna which takes advantage of the enhancement produced by the excitation of the various airframe polarities.
It is a still further object of the subject invention to provide a compact wave antenna which provides consistent reliable coupler tuning.